1. Field of the Invention
The present invention relates to a method of illuminating a liquid crystal layer and a liquid crystal display (hereinafter, LCD) using the same, and more particularly, to a method of illuminating a liquid crystal utilized in an LCD and capable of decreasing manufacturing costs as well as enhancing both brightness characteristics depending on visual angles and whole brightness characteristics, and an LCD using the illuminating method.
2. Description of the Related Art
Generally, an LCD is defined as one of flat panel displays which display data processed in an information processing device as characters, images and moving pictures using an optical property of liquid crystal in which light transmittance is varied depending on an intensity of an applied electric field.
Liquid crystal in an LCD controls transmittance of an incident light wave or beam depending on the intensity of an applied electric field to allow an image to be displayed. This means that all LCDs using such a liquid crystal require a light for displaying an image.
Such light is provided by an electrical energy internally charged in the LCD itself or from an external light source.
Where internally charged electrical energy is used in an LCD as a light source, an image can be displayed without being effected by an external environment of the LCD, such as existence or nonexistence of an external light source or intensity of light.
In the meantime, where the external light source is used for supplying light to an LCD, an image can be displayed with very small power consumption but the display is greatly affected by an external environment. For example, the display is not possible where there is no externally available light, or the quality of the display is greatly degraded where the intensity of an existing external light is insufficient.
Therefore, an LCD utilizing internally charged electrical energy for providing light to a liquid crystal layer, herein referred to as a xe2x80x9ctransmission type LCDxe2x80x9d, has been widely used.
The transmission type LCD generally includes an LCD panel for controlling a liquid crystal and various elements for effectively using light which is supplied to the LCD panel after being generated from a lamp.
Specifically, as shown in FIG. 1, a conventional transmission type LCD 80 includes an LCD panel 20 and a backlight assembly 40.
The LCD panel 20 includes a TFT substrate 16 having a selected effective display area, a color filter substrate facing the TFT substrate 16, a liquid crystal layer (not shown) interposed between the TFT substrate 16 and the color filter substrate 14, a lower polarizing plate 18 attached to an outer surface of the TFT substrate 16 and an upper polarizing plate 12 attached to an outer surface of the color filter substrate 14.
The LCD panel 20 requires an incident light having an optical distribution of a planar light source and a uniform brightness in order to display an image with a uniform quality throughout the entire area of the effective display area.
However, since an optical distribution of a planar light source is difficult to obtain as an incident light, the backlight assembly 40 is used in the LCD 80.
The backlight assembly 40 includes a lamp assembly, a light guiding plate 35, a reflection plate 38, a diffusion plate 34, a prism sheet 33, a reflection polarization film 32, and a reflection polarization film protection sheet 31.
The lamp assembly includes a CCFT (Cold cathode fluorescent tube) type lamp 37 for generating a white light that is similar to natural light and having a long life time and an easy manufacturing characteristic, and a lamp reflector 36 for directing the white light toward one direction.
Since the CCFT type lamp 37 generates light having an optical distribution of a linear light source, a member is required to transform the generated light into light having an optical distribution of a planar light source.
Specifically, in order to transform the light generated by the CCFT type lamp 37 having the optical distribution of the linear type into the light having the optical distribution of the planar type, the light guiding plate 35 is used.
The light guiding plate 35 has a plate-shaped parallelepiped structure. An incident light having the optical distribution of the linear light source is uniformly reflected throughout the entire area of the light guiding plate 35, and the optical distribution of the planar light source is thus obtained.
The light illuminated from the light guiding plate 35 has the optical distribution of the planar light source but it has a low uniformity of brightness. Therefore, it is difficult to display an image having a high quality using the light guiding plate 35 alone.
To overcome this drawback, the diffusion plate 34 for diffusing the light illuminated from the light guiding plate 35 is provided on the light guiding plate 35.
The light outputted from the diffusion plate 34 has an improved optical uniformity, however the light has an irregular progressive direction. Thus, a visual angle at the front side is significantly decreased.
To improve the visual angle at the front side, one sheet or two sheets of prism sheet 33 is disposed on the diffusion plate 34.
The light passes through the CCFT type lamp 37, the light guiding plate 35, the diffusion plate 34, the prism sheet 33, the lower polarizing plate 18, the TFT substrate 16, the liquid crystal layer, the color filter substrate 14, and the upper polarizing plate 12 to display an image.
The light that generated from the CCFT type lamp 37 has two kinds of wave forms, i.e., a P wave and an S wave. The P wave light passes through the lower polarizing plate 18 but the S wave light does not pass through the lower polarizing plate 18 and is dissipated.
Here, the P wave and the S wave are classified depending on the polarizing axis which the waves pass through. Particularly, the light that passes through the lower polarizing plate 18 is defined as the P wave and the light that does not pass through the lower polarizing plate 18 is defined as the S wave. This means that only 50% of the light output from the CCFT type lamp 37 arrives at a user""s eye.
To improve the low light efficiency, the reflection polarization film 32 is disposed on the prism sheet 33. The reflection polarization film 32 passes the P wave but it reflects the S wave and converts the reflected S wave into the P wave.
The reflection polarization film 32 is easily contaminated or scratched due to foreign particles and is susceptible to a Moire phenomenon due to a relationship between the prism sheet and a pattern of the TFT substrate. Also, the reflection polarization film 32 has a non-uniform visual angle.
In order to prevent these problems, the protection sheet 31 is disposed on the reflection polarization film 32.
The protection sheet 31 is usually made of a polycarbonate (PC)-based material, which is manufactured by an extrusion process into a form of a thin sheet. The protection sheet 31 has a vague polarizing axis as shown in FIG. 2. Therefore, the protection sheet 31 prevents a lowering of the brightness even if an angle between a polarizing axis of the reflection polarization film 32 and a polarizing axis of the protection sheet 31 is not adjusted, and it also remedies disadvantages of the reflection polarization film 32.
Although the protection sheet 31 made of the PC-based synthetic resin guarantees a certain level of brightness regardless of the polarizing axis, the film experiences a deformation such as a wrinkling and shrinkage due to a considerably high thermal expansion coefficient and increases the manufacturing cost of the LCD due to a high cost of the extrusion process.
Accordingly, in one aspect of the present invention, there is provided a method for illuminating a liquid crystal layer, which allows an image display with a high brightness while reducing overall manufacturing costs.
In another aspect, there is provided an LCD, which displays an image with a high brightness while reducing overall manufacturing costs.
According to the method for illuminating a liquid crystal layer, a light including a first light passing through a first polarizing axis and a second light passing through a second polarizing axis is generated. The optical properties of the first and second lights are changed, if required. More specifically, the polarization of the second light is changed such that the second light passes through the first polarizing axis, thereby enhancing brightness of light to be utilized for illuminating the liquid crystal layer. The first light and the changed second light are passed through a third polarizing axis which allows a brightness pattern repeated in a range of a first value and a second value depending on an angle between the third polarizing axis and the first polarizing axis, the first value being greater than the second value. The angle between the third polarizing axis and the first polarizing axis is controlled such that the first light and the changed second light are transmitted as a third light having a brightness of a desired value. The desired value is equal to or greater than the second value and equal to or less than the first value. The third light is provided to a liquid crystal layer.
In another aspect of the present invention, an LCD includes a light source generating a light including a first light passing through a first polarizing axis and a second light passing through a second polarizing axis. An optical distribution changing means changes the optical properties of the first and second lights. A reflection polarization film having the first polarizing axis through which the first light passes, changes the polarization of the second light into that of the first light so that the changed second light passes the reflection polarization film. A protection sheet is disposed on the reflection polarization film. The protection sheet has a third polarizing axis, and passes therethrough the first light and the changed second light to have a brightness pattern repeated in a range of a first value and a second value depending on an angle between the third polarizing axis and the first polarizing axis, the first value being greater than the second value. The angle is adjusted such that the first light and the changed second light are transmitted as a third light having a brightness of a desired value equal to or greater than the second value and equal to or less than the first value. An LCD panel receives the third light and then displays an image.
According to the present invention, the protection sheet is manufactured by an extension process of PET-based material and disposed on the reflection polarization film used for enhancing the display brightness. The protection sheet of the present invention can be manufactured at a low price compared with the conventional protection sheet manufactured by an extrusion process of PC-based material. Also, since the protection sheet of the present invention has a relatively low thermal expansion coefficient compared with that of the conventional protection sheet, deformation of the protection sheet is prevented, color variation depending on variation of the visual angle is minimized, etc.